Microbial fuel cells (MFCs) represent a key technology in microbial electrochemistry, enabling the direct conversion of chemical energy from organic compounds into electrical energy through microbial metabolism. While their potential for wastewater treatment and energy recovery has been widely demonstrated, practical applications are often constrained by suboptimal environmental conditions, such as extreme temperature, pH, salinity, or nutrient limitations. Recent research has highlighted the crucial role of extremophiles in overcoming these challenges, as their unique physiological and molecular adaptations allow efficient extracellular electron transfer under harsh conditions. This chapter reviews the principles of MFC operation, the diversity of electron transfer mechanisms—including direct contact, conductive appendages, and electron shuttles—and recent advances in understanding electricity generation by thermophiles, psychrophiles, acidophiles, alkaliphiles, halophiles, and other extremophiles. The application of extremophiles in MFCs not only enhances system performance and broadens technological versatility but also opens avenues for biosensing, bioremediation, and the development of biodegradable conductive materials relevant to e-waste mitigation. Despite substantial progress, the molecular basis of extracellular electron transfer in many extremophiles remains largely unexplored. Future discoveries are expected to expand the scope of microbial electrochemistry and accelerate the integration of MFCs into sustainable energy and environmental technologies.